Jeremy M. Reimers

Formation of accumbens GluR2-lacking AMPA receptors mediates incubation of cocaine craving

Relapse to cocaine use after prolonged abstinence is an important clinical problem. This relapse is often induced by exposure to cues associated with cocaine use. To account for the persistent propensity for relapse, it has been suggested that cue-induced cocaine craving increases over the first several weeks of abstinence and remains high for extended periods. We and others identified an analogous phenomenon in rats that was termed ‘incubation of cocaine craving’: time-dependent increases in cue-induced cocaine-seeking over the first months after withdrawal from self-administered cocaine. Cocaine-seeking requires the activation of glutamate projections that excite receptors for α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) in the nucleus accumbens. Here we show that the number of synaptic AMPA receptors in the accumbens is increased after prolonged withdrawal from cocaine self-administration by the addition of new AMPA receptors lacking glutamate receptor 2 (GluR2). Furthermore, we show that these new receptors mediate the incubation of cocaine craving. Our results indicate that GluR2-lacking AMPA receptors could be a new target for drug development for the treatment of cocaine addiction. We propose that after prolonged withdrawal from cocaine, increased numbers of synaptic AMPA receptors combined with the higher conductance of GluR2-lacking AMPA receptors causes increased reactivity of accumbens neurons to cocaine-related cues, leading to an intensification of drug craving and relapse.

Cell Surface AMPA Receptors in the Rat Nucleus Accumbens Increase during Cocaine Withdrawal But Internalize after Cocaine Challenge in Association with Altered Activation of Mitogen-Activated Protein Kinases

Although some studies report increased responsiveness of nucleus accumbens (NAc) AMPA receptors (AMPARs) after withdrawal from repeated cocaine treatment, others report decreased responsiveness after withdrawal plus cocaine challenge. Here we examine this apparent contradiction by quantifying cell surface and intracellular AMPAR subunits in the NAc before and after a challenge injection in behaviorally sensitized rats. Because MAPKs (mitogen-activated protein kinases) regulate AMPAR trafficking and are implicated in addiction, we also evaluated phosphorylation of extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38. Glutamate receptor 1 (GluR1) and GluR2 surface/intracellular (S/I) ratios were increased after 14 d of withdrawal in sensitized rats but were decreased 24 h after challenge with cocaine (which elicited a sensitized locomotor response) or saline (which elicited conditioned locomotion). These findings suggested redistribution of GluR1/2-containing receptors, a possibility supported by immunoprecipitation experiments indicating that most AMPARs in the NAc are GluR1/2 or GluR2/3, with few homomeric GluR1 or GluR1/3 receptors. In sensitized rats, ERK phosphorylation in the NAc increased during withdrawal and normalized after cocaine challenge. JNK phosphorylation also increased after withdrawal, but after cocaine challenge, it was inversely related to GluR1 and GluR2 S/I ratios. After saline challenge, p38 phosphorylation was increased. In summary, surface expression of GluR1/2-containing AMPARs increased in the NAc of sensitized rats, but AMPARs internalized after a single reexposure to cocaine or cocaine-related cues. ERK phosphorylation paralleled AMPAR surface expression. Although JNK results were complex, JNK and p38 may be involved in AMPAR internalization after cocaine or saline challenge, respectively.

Loss of Predominant Shank3 Isoforms Results in Hippocampus-Dependent Impairments in Behavior and Synaptic Transmission

The Shank3 gene encodes a scaffolding protein that anchors multiple elements of the postsynaptic density at the synapse. Previous attempts to delete the Shank3 gene have not resulted in a complete loss of the predominant naturally occurring Shank3 isoforms. We have now characterized a homozygous Shank3 mutation in mice that deletes exon 21, including the Homer binding domain. In the homozygous state, deletion of exon 21 results in loss of the major naturally occurring Shank3 protein bands detected by C-terminal and N-terminal antibodies, allowing us to more definitively examine the role of Shank3 in synaptic function and behavior. This loss of Shank3 leads to an increased localization of mGluR5 to both synaptosome and postsynaptic density-enriched fractions in the hippocampus. These mice exhibit a decrease in NMDA/AMPA excitatory postsynaptic current ratio in area CA1 of the hippocampus, reduced long-term potentiation in area CA1, and deficits in hippocampus-dependent spatial learning and memory. In addition, these mice also exhibit motor-coordination deficits, hypersensitivity to heat, novelty avoidance, altered locomotor response to novelty, and minimal social abnormalities. These data suggest that Shank3 isoforms are required for normal synaptic transmission/plasticity in the hippocampus, as well as hippocampus-dependent spatial learning and memory.

The Role of Glutamate Receptor Redistribution in Locomotor Sensitization to Cocaine

α-Amino-3-hydroxy-5-methylisoxazole-4-propionate receptor (AMPAR) surface expression in the nucleus accumbens (NAc) is enhanced after withdrawal from repeated cocaine exposure. However, it is unclear whether this contributes to the expression of locomotor sensitization and whether similar changes can be observed in other striatal regions. In this study we examined the relationship between AMPAR surface expression in the NAc and locomotor sensitization. We also examined AMPAR distribution in the dorsolateral striatum (DS) and NMDA receptor (NMDAR) distribution in the NAc and DS. Trends but no significant changes in NMDAR distribution were found in the NAc after withdrawal. No NMDAR changes were observed in the DS. AMPAR surface expression was increased in the NAc 15 days after the last exposure to cocaine, but decreased in the DS. Re-exposure to cocaine on withdrawal day 14 decreased AMPAR surface expression in the NAc 24 h, but not 30 min, after challenge, but increased it in the DS 24 h and 30 min after challenge. Locomotor sensitization was evaluated at times associated with increased or decreased AMPAR surface expression in the NAc. The magnitude of sensitization did not vary with changes in the level of AMPAR surface expression, nor was it significantly reduced by decreasing AMPAR transmission through intra-NAc infusion of CNQX before cocaine challenge. On the basis of our results, and other findings, we suggest that the expression of sensitization has no clear relationship to altered AMPAR surface expression in the NAc, although the latter may have a role in the enhanced pursuit and self-administration of drugs observed in sensitized rats.

Quantitative analysis of AMPA receptor subunit composition in addiction-related brain regions

The subunit composition of α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPARs) is an important determinant of AMPAR biophysical properties and trafficking. To date, AMPAR subunit composition has been quantitatively evaluated only for the hippocampus, where different experimental approaches have yielded different results. Here, we used quantitative co-immunoprecipitation to characterize GluA1-3 associations in the adult rat nucleus accumbens, dorsal striatum, prefrontal cortex, and hippocampus, and blue native electrophoresis (BNE) to study GluA1-3 assembly state. In all brain regions, co-immunoprecipitation experiments showed that ~90% of GluA1 was associated with GluA2 or GluA3 (most was GluA1A2). All regions contained a small number of GluA1A3 receptors. Homomeric GluA1 receptors may also exist. More than half of the GluA2 (53%-65% depending on the region) was not associated with GluA1. However, this represents an over-estimate of the percent of GluA2 present in GluA2A3 receptors, based on BNE results demonstrating that the majority of GluA2 exists as dimers, rather than functional tetrameric receptors. Relatively more GluA1 was present in tetramers. Together with other findings, our results suggest a dominant role for GluA1A2 receptors in all brain regions examined. They also help explain why different results for hippocampal AMPAR subunit composition were obtained using co-immunoprecipitation, which assesses the total cellular pool of AMPARs including partially assembled AMPARs in intracellular compartments, and electrophysiological approaches, which can selectively assess tetrameric (functional) AMPARs on the cell surface.

Autism-Associated Insertion Mutation (InsG) of Shank3 Exon 21 Causes Impaired Synaptic Transmission and Behavioral Deficits

SHANK3 (also known as PROSAP2) is a postsynaptic scaffolding protein at excitatory synapses in which mutations and deletions have been implicated in patients with idiopathic autism, Phelan–McDermid (aka 22q13 microdeletion) syndrome, and other neuropsychiatric disorders. In this study, we have created a novel mouse model of human autism caused by the insertion of a single guanine nucleotide into exon 21 (Shank3G). The resulting frameshift causes a premature STOP codon and loss of major higher molecular weight Shank3 isoforms at the synapse. Shank3G/G mice exhibit deficits in hippocampus-dependent spatial learning, impaired motor coordination, altered response to novelty, and sensory processing deficits. At the cellular level, Shank3G/G mice also exhibit impaired hippocampal excitatory transmission and plasticity as well as changes in baseline NMDA receptor-mediated synaptic responses. This work identifies clear alterations in synaptic function and behavior in a novel, genetically accurate mouse model of autism mimicking an autism-associated insertion mutation. Furthermore, these findings lay the foundation for future studies aimed to validate and study region-selective and temporally selective genetic reversal studies in the Shank3G/G mouse that was engineered with such future experiments in mind.

Altered Striatal Synaptic Function and Abnormal Behaviour in Shank3 Exon4‐9 Deletion Mouse Model of Autism

Shank3 is a multi‐domain, synaptic scaffolding protein that organizes proteins in the postsynaptic density of excitatory synapses. Clinical studies suggest that ∼0.5% of autism spectrum disorder (ASD) cases may involve SHANK3 mutation/deletion. Patients with SHANK3 mutations exhibit deficits in cognition along with delayed/impaired speech/language and repetitive and obsessive/compulsive‐like (OCD‐like) behaviors. To examine how mutation/deletion of SHANK3 might alter brain function leading to ASD, we have independently created mice with deletion of Shank3 exons 4‐9, a region implicated in ASD patients. We find that homozygous deletion of exons 4‐9 (Shank3e4‐9 KO) results in loss of the two highest molecular weight isoforms of Shank3 and a significant reduction in other isoforms. Behaviorally, both Shank3e4‐9 heterozygous (HET) and Shank3e4‐9 KO mice display increased repetitive grooming, deficits in novel and spatial object recognition learning and memory, and abnormal ultrasonic vocalizations. Shank3e4‐9 KO mice also display abnormal social interaction when paired with one another. Analysis of synaptosome fractions from striata of Shank3e4‐9 KO mice reveals decreased Homer1b/c, GluA2, and GluA3 expression. Both Shank3e4‐9 HET and KO demonstrated a significant reduction in NMDA/AMPA ratio at excitatory synapses onto striatal medium spiny neurons. Furthermore, Shank3e4‐9 KO mice displayed reduced hippocampal LTP despite normal baseline synaptic transmission. Collectively these behavioral, biochemical and physiological changes suggest Shank3 isoforms have region‐specific roles in regulation of AMPAR subunit localization and NMDAR function in the Shank3e4‐9 mutant mouse model of autism. Autism Res 2016, 9: 350–375.

BDNF contributes to both rapid and homeostatic alterations in AMPA receptor surface expression in nucleus accumbens medium spiny neurons

Brain‐derived neurotrophic factor (BDNF) plays a critical role in plasticity at glutamate synapses and in the effects of repeated cocaine exposure. We recently showed that intracranial injection of BDNF into the rat nucleus accumbens (NAc), a key region for cocaine addiction, rapidly increases α‐amino‐3‐hyroxy‐5‐methyl‐4‐isoxazole‐propionic acid receptor (AMPAR) surface expression. To further characterize BDNFs role in both rapid AMPAR trafficking and slower, homeostatic changes in AMPAR surface expression, we investigated the effects of acute (30 min) and long‐term (24 h) treatment with BDNF on AMPAR distribution in NAc medium spiny neurons from postnatal rats co‐cultured with mouse prefrontal cortex neurons to restore excitatory inputs. Immunocytochemical studies showed that acute BDNF treatment increased cell surface GluA1 and GluA2 levels, as well as their co‐localization, on NAc neurons. This effect of BDNF, confirmed using a protein crosslinking assay, was dependent on ERK but not AKT signaling. In contrast, long‐term BDNF treatment decreased AMPAR surface expression on NAc neurons. Based on this latter result, we tested the hypothesis that BDNF plays a role in AMPAR ‘scaling down’ in response to a prolonged increase in neuronal activity produced by bicuculline (24 h). Supporting this hypothesis, decreasing BDNF signaling with the extracellular BDNF scavenger TrkB‐Fc prevented the scaling down of GluA1 and GluA2 surface levels in NAc neurons normally produced by bicuculline. In conclusion, BDNF exerts bidirectional effects on NAc AMPAR surface expression, depending on duration of exposure. Furthermore, BDNFs involvement in synaptic scaling in the NAc differs from its previously described role in the visual cortex.

Novel Shank3 mutant exhibits behaviors with face validity for autism and altered striatal and hippocampal function

Mutations/deletions in the SHANK3 gene are associated with autism spectrum disorders and intellectual disability. Here, we present electrophysiological and behavioral consequences in novel heterozygous and homozygous mice with a transcriptional stop cassette inserted upstream of the PDZ domain‐coding exons in Shank3 (Shank3E13). Insertion of a transcriptional stop cassette prior to exon 13 leads to loss of the two higher molecular weight isoforms of Shank3. Behaviorally, both Shank3E13 heterozygous (HET) and homozygous knockout (KO) mice display increased repetitive grooming, deficits in social interaction tasks, and decreased rearing. Shank3E13 KO mice also display deficits in spatial memory in the Morris water maze task. Baseline hippocampal synaptic transmission and short‐term plasticity are preserved in Shank3E13 HET and KO mice, while both HET and KO mice exhibit impaired hippocampal long‐term plasticity. Additionally, Shank3E13 HET and KO mice display impaired striatal glutamatergic synaptic transmission. These results demonstrate for the first time in this novel Shank3 mutant that both homozygous and heterozygous mutation of Shank3 lead to behavioral abnormalities with face validity for autism along with widespread synaptic dysfunction. Autism Res 2017, 10: 42–65.

Trafficking of calcium-permeable and calcium-impermeable AMPA receptors in nucleus accumbens medium spiny neurons co-cultured with prefrontal cortex neurons

&NA; AMPA receptor (AMPAR) transmission onto medium spiny neurons (MSNs) of the adult rat nucleus accumbens (NAc) is normally dominated by GluA2‐containing, Ca2+‐impermeable AMPAR (CI‐AMPARs). However, GluA2‐lacking, Ca2+‐permeable AMPA receptors (CP‐AMPARs) accumulate after prolonged withdrawal from extended‐access cocaine self‐administration and thereafter their activation is required for the intensified (incubated) cue‐induced cocaine craving that characterizes prolonged withdrawal from such regimens. These findings suggest the existence of mechanisms in NAc MSNs that differentially regulate CI‐AMPARs and CP‐AMPARs. Here, we compared trafficking of GluA1A2 CI‐AMPARs and homomeric GluA1 CP‐AMPARs using immunocytochemical assays in cultured NAc MSNs plated with prefrontal cortical neurons to restore excitatory inputs. We began by evaluating constitutive internalization of surface receptors and found that this occurs more rapidly for CP‐AMPARs. Next, we studied receptor insertion into the membrane; combined with past results, the present findings suggest that activation of protein kinase A accelerates insertion of both CP‐AMPARs and CI‐AMPARs. We also studied constitutive cycling (net loss of receptors from the membrane under conditions where internalization and recycling are both occurring). Interestingly, although CP‐AMPARs exhibit faster constitutive internalization, they cycle at similar rates as CI‐AMPARs, suggesting faster reinsertion of CP‐AMPARs. In studies of synaptic scaling, long‐term (24 h) activity blockade increased surface expression and cycling rates of CI‐AMPARs but not CP‐AMPARs, whereas long‐term increases in activity produced more pronounced scaling down of CI‐AMPARs than CP‐AMPARs but did not alter receptor cycling. These findings can be used to evaluate and generate hypotheses regarding AMPAR plasticity in the rat NAc following cocaine exposure. HighlightsSurface CP‐AMPARs on MSNs constitutively internalize more rapidly than CI‐AMPARs.Activation of PKA accelerates insertion of both CP‐AMPARs and CI‐AMPARs.CI‐AMPARs and CP‐AMPARs cycle at similar rates.Activity blockade (24 h) increases surface expression and cycling rates of CI‐AMPARs.Increased activity (24 h) mainly scales down CI‐AMPARs and does not alter cycling.